1. Field of the Invention
This invention relates to an image scanner for reading a given objective image and outputting image data to image processing devices such as a computer, and more particularly, to an image scanner in which appropriate image scanning and reading for image processing can be carried out by use of a rod-type light source capable of emitting intense light without being influenced by undesirable heat generated by the light source.
2. Description of the Prior Art
Recently image scanners (image scanning and reading devices) are being increasingly used for the purpose of optically reading a given objective image by use of an image sensor such as a CCD line sensor and outputting image signals to various image processing devices including a computer, a copying machine and a facsimile. A fluorescent lamp which is generally used as a light source in the image scanner has suffered a disadvantage that the intensity of illumination around both end portions of the fluorescent glass tube of the lamp is remarkably reduced during prolonged use, consequently to make the distribution of light uneven. Besides, the intensity of illumination of the fluorescent lamp disadvantageously varies with the change of ambient temperature.
Furthermore, because the fluorescent lamp cannot radiate intense light, it is not applicable in a color image scanner as proposed by, for example, Japanese Utility Model Application Disclosure SHO 62-161460(A), in which a given color objective image is optically read by decomposing the given image into the three primary colors by use of three color filters. That is to say, in the color image scanner, as schematically illustrated in FIG. 1, a beam of image light b obtained by illuminating a given color objective image i with light from light sources L is directed to a color filter unit f via reflectors r and introduced into an image sensor s. In some cases, the image light b from the objective image i passes through an infrared-cut filter c for removing undesirable infrared ray component from the image light b. Accordingly, the image light b reaching the image sensor s is considerably reduced. Because of this, use of the fluorescent lamp entails problems such as lowering of the image scanning speed and deterioration of the quality of a reproduced image resultingly obtained.
It is possible in the image scanner of this type to employ a rod-type light source as proposed in Japanese Patent Application Public Disclosure SHO 62-142465(A) in place of the aforementioned fluorescent lamp. This known rod-type light source comprises a transparent rod with a diffusion stripe extending in the longitudinal direction of the rod and a light source lamp disposed at one end of the rod. By operating the light source lamp to emit light into the rod, the light which propagates inside the rod is diffused by the diffusion stripe and consequently radiate outside. Since the rod-type light source can employ a halogen incandescent lamp which emits intense light, it is available for image scanning of a color image.
In the image scanner, it is preferable to locate a light source as close as possible to a transparent reading platen on which a given original image sheet to be scanned is placed. This is because the intensity of the light impinging on the given image sheet is in inverse proportion to a square of the distance between the light source and the given image sheet. However, the halogen incandescent lamp is generally enveloped in a cup-like reflector in order to produce desirable intense light and therefore becomes large in overall size, though the transparent rod attached to the lamp is comparatively small in diameter. Therefore, this rod-type light source cannot be placed close to the reading platen, and the distance between the transparent rod from which the light is emitted and the reading platen could not be decreased. Thus, the conventional image scanner using the rod-type light source has been inferior in light efficiency and space factor.
In a case of using a high luminescent lamp such as the halogen incandescent lamp as a light source, some apprehension is caused by the influence of undesirable heat generated by the lamp on the component parts of the image scanner and the given original sheet to be read on the reading platen. For instance, as illustrated in FIG. 1, the infrared-cut filter c for eliminating thermal energy from the image light b is disposed at a distance from the light source L, so that the thermal energy contained in the image light b is mostly given forth when the image light b is propagating from the light source L to the image sensor S. Thus, the conventional image scanner has not sufficiently utilized the effect of the infrared-cut filter incorporated therein.